Project Summary The most important function of vitamin D is the regulation of intestinal calcium absorption ? a classical role of vitamin D critical for bone health. Despite the importance of vitamin D in the intestine, the mechanisms of vitamin D action in intestine are not clear. Although most Ca absorption research has focused on the proximal small intestine, our novel preliminary data using distal intestine specific knockout (KO) or transgenic mice reveal an essential role for VDR in the distal intestine. The proposed research employs a genomic approach coupled to physiology studies in novel mouse models to test the hypothesis that the proximal and distal segments of the intestine have unique regulatory pathways controlling VDR expression and vitamin D action, and that calcium absorption and expression of VDR target genes in the distal intestine contribute significantly to bone and mineral metabolism. To address this hypothesis we have developed two specific aims. Aim 1: Define the functional genomics of VDR expression and signaling that mediate calcium absorption in the proximal and distal intestine and its impact on whole body calcium and bone metabolism. Using a genome-wide approach we will identify spatial (proximal vs distal intestine; villus vs crypt) and temporal (pre vs post weaning; young vs adult) effects on chromatin architecture of the Vdr gene locus to identify critical upstream regulators of Vdr expression in proximal and distal intestine. Next, VDR target genes in the proximal and distal intestine and in progenitor vs differentiated cells will be identified on a genome-wide scale. The human relevance of VDR targets as well as VDR regulatory mechanisms will be determined in studies using human-derived organoid cultures. Finally, we will test the hypothesis that Ca absorption in the distal intestine contributes significantly to bone and mineral metabolism using unique animal models including VDR KO mice with transgenic expression of VDR only in the distal intestine. We will also determine for the first time the effect of VDR deletion throughout the intestine or specifically in the distal intestine in adult mice on calcium and bone metabolism. Aim 2: Determine the translational potential of targeting the distal intestine to improve calcium balance and protect bone. We will test whether targeting the absorptive capacity of the distal intestine can be an effective way to maximize Ca absorption and minimize adult bone loss. This will be done using transgene-enhanced expression of VDR in the distal intestine and use of a natural, distal intestine-targeted vitamin D form (25 ? glucuronide 1,25(OH)2D3) to improve calcium balance and protect against bone loss. This proposed research is responsive to PA-16-332 ?Nutrigenetics and Nutrigenomics Approaches for Nutrition Research.? It combines expertise in vitamin D biology, nutrition, and genomics to determine how the molecular actions of vitamin D can be utilized to improve Ca status in groups at risk for accelerated bone loss and osteoporosis.